Thermodynamic Mechanism Evaluate the Feasibility of Oil 1 Shale Pyrolysis by Topochemical Heat 2

: Topochemical heat in-situ pyrolysis of oil shale is achieved by injecting high 9 temperature nitrogen to promote oil shale pyrolysis and release heat, and then injecting air to 10 trigger oil shale combustion in the early stage of oil shale pyrolysis, and then by injecting 11 normal temperature air continuously to promote local oxidation of oil shale in the later stage. 12 In order to verify the oil and gas recovery by topochemical heat method, Jilin University has 13 chosen Fuyu City, Jilin Province, to carry out pilot project of oil shale in-situ pyrolysis by 14 topochemical heat method. Besides, in order to infer the spontaneity, feasibility and difficulty 15 of continuous pyrolysis of oil shale based on topochemical heat, this paper, the mechanism of 16 solid-state pyrolysis and the thermodynamic analysis of transition state of oil shale in Fuyu area 17 are discussed. Because the second stage of oil shale pyrolysis is the main stage of oil production. 18 Therefore, the characteristics of Gibbs free energy, free enthalpy and free entropy of transition 19 state in the main oil production stage of oil shale pyrolysis are obtained by calculation. The 20 results show that in situ pyrolysis of oil shale topochemical heat can be carried out 21 spontaneously and continuously, and the release characteristics of volatiles during pyrolysis of 22 oil shale are described.


26
Oil shale is an immature hydrocarbon-generating medium. It can produce oil and gas only 27 when the temperature reaches the pyrolysis temperature of kerogen [1] . The traditional pyrolysis 28 mode of oil shale is mainly surface retorting process. Because surface retorting needs to 29 establish surface factories, the one-time economic input cost is high. In addition, surface 30 The pyrolysis process of oil shale is a multiphase and multistage coupled chemical reaction. and showed that the thermal decomposition mechanism of Moroccan Rif region's primary oil 53 shale can be described by n-order (n=1.550),and the activation energy was calculated by 54 Kissinger-Akahira-Sunose(KAS)method. In addition, the transition state free Gibbs energy ΔG ≠ , 55 free enthalpy ΔH ≠ and free entropy ΔS ≠ indicated that the pyrolysis of oil shale is non-56 spontaneous at low temperature [6] . Kuang et al. studied the carbon oxidation and pyrolysis 57 process of Green River oil shale by non-isothermal thermogravimetric analysis. The extensive 58 applicability of solid-state pyrolysis mechanism was verified by various kinetic calculation 59 4 monitoring wells of formation temperature and pressure changes, and FK-3 is a water level 89 monitoring well for real-time monitoring of water level change in gas seepage control Area. 90 91

Fig. 1 Location of oil shale in-situ pyrolysis region and layout of wells 92
Among them, two wells were drilled without coring, and only FK-3 well were drilled with 93 coring. After analyzing the core of FK-3 well, it is found that the burial depth of Fuyu oil shale 94 is 477-486 meters. Under this burial depth, two core samples of FK-3 Well are selected for 95 industrial analysis, element analysis and Fisher analysis, which are located on the roof and floor 96 of oil shale reservoir respectively. The samples were ground before the test. In order to avoid 97 the influence of different particle sizes on the test results, the grinded oil shale samples were 98 sieved into uniform particle sizes. Dry in a constant temperature drying oven at 60 ℃ to a 99 constant weight. The results are shown in Table 1. 100

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The weight of samples was controlled at 9.0(±0.2 mg), in which the initial temperature of 103 TG curve was 25℃, the heating rate was 10, 20, 30, 40, 50 ℃/min, and the reaction termination 104 temperature was 900℃. The purge gas is nitrogen and air with flow rate of 60 mL/min and the 105 protective gas is high purity nitrogen with flow rate of 25 mL/min. In order to reproduce and 106 reproduce the experiment, each group of experiments was repeated at least twice. The TG-DTG 107 curve of Fuyu oil shale pyrolysis in nitrogen and air was shown in Figure 2. According to thermogravimetric curves, oil shale pyrolysis can be divided into three stages, 111 but the main oil generation stage is the second stage of oil shale pyrolysis. In the second reaction 112 stage. Temperature ranges from 300 to 620 ℃, and the weight loss rate of oil shale is about 15% 113 to 17% in this stage. With the increase of heating rate, the pyrolysis process of oil shale becomes 114 more intense. According to proximate analysis data, the first stage of oil shale pyrolysis was 115 the process of free water loss, and the weight loss rate of this process is about 1.2%~1.6%. The 116 third stage was mainly the decomposition process of clay minerals under high temperature. The 117 weight loss rate of this process is slow, about 2%~2.5%. 118 In addition, compared with nitrogen atmosphere, the temperature range of kerogen 119 pyrolysis in the second stage of oil shale pyrolysis is broadened in air atmosphere. According 120 to the DTG curve, the second stage of oil shale pyrolysis in nitrogen atmosphere has a higher 121 degree of aggregation, and the temperature change has a greater impact on the volatile release 122 rate. However, in the air atmosphere, the release rate of volatile matter is relatively stable, and 123 the maximum pyrolysis rate is also reduced to 0.1 compared with 0.21 of nitrogen, showing a 124 uniform and slow release process throughout the second stage. This is because the oxygen in 125 the air has a combustion supporting effect, which is a multiphase reaction synchronous 126 propulsion process. As shown in Figure 3, the organic matter and volatile matter in oil shale 127 are burned in oxygen. The heat released by the reaction will lead to the increase of ambient 128 temperature around oil shale particles, which will promote the temperature increase of 129 surrounding particles, and promote the release of volatile matter and the combustion of organic 130 matter. Therefore, the temperature range of the second stage has been widened in air, and the 131 release rate of organic matter becomes slow and uniform. shale. When the temperature increases to 450℃, the diffraction peaks of Kaolinite, Illite 136 /Smectite mixed layer and pyrite decrease in varying degrees [27] . When the temperature rises 137 to 750 ℃, the above clay minerals decompose completely.

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The process of solving the kinetic parameters is to determine the activation energy and 143 frequency factors that affect the reaction rate during the reaction process, and then reveal the 144 control factors of the reaction, find the regularity of the reaction and realize the optimization of 145 the actual process. The pyrolysis process of oil shale is a multiphase and multi-stage chemical 146 reaction [22] . The simple model matching process does not accurately describe the complex 147 heterogeneous pyrolysis system [23] . International Confederation for Thermal Analysis and 148 Calorimetry (ICTAC) points out that the model free function method is more suitable for 149 solving the dynamics of solid fuels [24] . In the kinetic analysis of the model free function method, 150 KAS, FWO, Friedman is three most commonly used methods [24][25] . Therefore, the three 151 methods were used to solve the pyrolysis kinetic parameters of Fuyu oil shale. 152

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KAS method is the abbreviation of Kissinger-Akahira-Sunose, and its calculation method 154 is as follows [9] . 155 Where, α-Conversion rate of oil shale pyrolysis in the second stage, %; m 0 -Initial mass 158 of oil shale at the second pyrolysis stage, mg; m-sample mass at T K, mg; m ꝏ -Final mass of 159 oil shale at the second pyrolysis stage, mg; β-Heating rate, K·min -1 ; T -reaction temperature, 160 K; R-gas constant, 8.314J·mol -1 ; G(α) -Integral form of the most probable mechanism function; 161 A-preexponential factor, s -1 ; E a -activation energy, kJ·mol -1 . 162 It can be seen from equation (2) that ln 2 is a primary function of-1 . By drawing ln 2 -163 (-1 ) at different conversion rates, the slope k can be obtained by fitting the curve with first order 164 function. The activation energy E a at the corresponding conversion rate can be obtained by 165 k=E a /RT. 166

167
In order to further verify the accuracy of KAS method, the Flynn-Wall-Ozawa method was 168 used to calculate the apparent activation energy E b again [10] . The approximate formula of Doyle 169 temperature integral was used, as shown in equation (3)-(4). 170 The temperature approximation is introduced into the pyrolysis integral equation (5). 173 The calculation formula of FWO method can be obtained, as shown in equation (6). 175 As can be seen from equation (6), lgβ is a first-order function of -1 . Through drawing lgβ 177 -(-1 ) at different conversion rates, the slope k can be obtained by fitting the curve with the first-178 order function. The activation energy E b can be calculated by k=0.4576E b /RT. 179

180
According to Arrhenius's law, the rate of reaction can be expressed as: 181 The logarithm of the two sides of the equation (1) is performed at the same time. 183 It can be seen from equation (2) that ln (dx/dt) -(-1/T) curves are made for different 185 conversion rates respectively. Slope k and intercept D. are obtained respectively, too. Activation 186 energy E c and pre-exponential factor A were calculated according to slope k and intercept D, 187 respectively. 188

Inference of the most probable mechanism function from y(α)-α curve 189
The values of data α i , y(α) i (i=0.05,0.1……0.95) and a=0.5,y(0.5) are brought into 41 190 groups of main functions of g(α) and corresponding f(α). The main function was calculated by 191 y(α) [12] , and the curve of y(a)-a is made, which is regarded as the standard curve. shows that the pyrolysis mechanism function of Fuyu oil shale satisfies the reaction mechanism 207 of random growth and subsequent nucleation. However, according to Figure 5 (b), when the 208 conversion is 20%-40% and 70%-90%, the dispersion between the test curve and the standard 209 curve is greater. This is mainly reflected in the high heating rate corresponds to the low 210 conversion interval, and the low heating rate corresponds to the high conversion interval. It may 211 be that after random nucleation, small molecules of different chemical bond types are received 212 at the growth stage. And because of the delayed heat transfer, the heat conduction products are 213 released centrally, and secondary pyrolysis occurs after nucleation. Besides, for the Johnson-214 Mehl-Avrami reaction model, there are many different kinetic indices n. The pre-exponential 215 factor A and other thermodynamic parameters can be calculated only if the kinetic exponent n 216 is known exactly. 217 For Johnson-Mehl-Avrami reaction mode [14] , 218 Combining with DTG curve, when the reaction rate reaches the maximum value, 221 In the formula, is the corresponding conversion at the peak of DTG curve, %. 224 Therefore, the kinetic exponent n can be obtained by combining the above three equations 225 In the formula, is the activation energy corresponding to the peak value of DTG curve, 229 is the temperature corresponding to the peak value of DTG curve, K; 230 According to Luke approximation [13] , 231 π( ) = 3 +18 2 +86 +96 4 +20 3 +120 2 +240 +120 (17)

232
In summary, the kinetic exponent n is calculated as shown in Table 2. 233  As shown in Figure 6, the extrapolation method is used to determine the pyrolysis 266 parameters of oil shale in the second stage. The initial loss temperature T O , ignition temperature 267 T I , maximum weight loss temperature T P , maximum weight loss rate (dm/dt) max and burnout 268 temperature Tc are obtained as shown in Table 3. 269

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The KAS method, the FWO method and the Friedman method are used to calculate the 272 kinetic curves of oil shale pyrolysis at the second stage, as shown in Figure 7 to 9 below. When 273 the KAS method and the FWO method are used, the correlation coefficient r is greater than 274 0.98. But in the process of calculation by the Freidman method, the correlation coefficient is 275 between 0.94 and 0.99, and the fitting degree of data is lower than that of KAS and FWO 276 methods. In addition, the data fluctuate greatly during the fitting process, which indicates that 277 When the heating rate is low, the area where oil shale pyrolysis is concentrated is between 285 40% and 80%, and the corresponding temperature is between 688 and 774K. When the 286 conversion rate is 80%, the maximum weight loss rate is reached. With the increase of heating 287 rate, the area where oil shale pyrolysis is concentrated gradually moves to the area where 288 conversion rate is 45%~95%. This is due to the hysteresis of heat transfer. Oil shale belongs to 289 heterogeneous system. In the process of pyrolysis, there is an active region of topo-reaction at 290 the interface between the reactant and the product. The energy provided by the system for this 291 area is larger than the activation energy of oil shale pyrolysis, so the oil shale in the active area 292 pyrolysis first. With the continuous progress of pyrolysis, the oil shale in the topo-active region 293 undergoes the random growth of products and the subsequent interfacial reaction of nucleation. 294 The active region of the reaction interface is gradually advancing toward the direction of high 295 conversion. According to the calculation results of the three methods, the activation energy of 296 Fuyu oil shale pyrolysis at the second stage was obtained as shown in the table 4 below. 297  Fig. 10 Curve of activation energy varies with conversion 300 The second stage of oil shale pyrolysis is the main oil production stage. The activation 301 energy of this stage is of great significance to the whole process and industrial production. The 302 second stage conversion rate of oil shale pyrolysis is calculated based on Thermogravimetric 303 curves. Since the activation energy is affected by the conversion rate, the corresponding 304 activation energy is calculated every 5% of the conversion rate. The activation energy of oil 305 shale pyrolysis at the second stage calculated by the three methods is shown in Figure 10 [26] . When 314 the conversion rate is higher than 80%, the activation energy increases sharply. This is because 315 the higher the conversion, the harder the reaction will occur. The activation energies obtained 316 by KAS and FWO methods are relatively stable. The average activation energies of the second 317 stage are 128.27 kJ·mol -1 、133.59kJ·mol -1 respectively, when the conversion is 40%~80%. 318 However, the activation energy of oil shale pyrolysis at the second stage obtained by Friedman 319 method reaches 158.23 kJ·mol -1 , and fluctuates greatly with the increase of conversion rate. 320

Thermodynamic decomposition characteristics 321
According to the transition state theory, kerogen needs to pass through a transition state in 322 the pyrolysis process of oil shale, and then to produce oil and gas. In this transition process, it 323 involves the complex coupling of multi-phase, multi-stage and multi-physical fields, and also 324 includes the redistribution of energy and the rearrangement of chemical bonds. Thermodynamic The Gibbs free energy ΔG ≠ is the expression of the work done by the chemical reaction to 334 the environment. As shown in Figure 11, in the oil shale pyrolysis system, the work done by 335 kerogen pyrolysis to the environment is the value of heat. That is to say, when the Gibbs free 336 energy ΔG ≠ of oil shale pyrolysis in nitrogen is greater than the activation energy E in air, the 337 pyrolysis reaction of oil shale in air can be excited. At the same time, the Gibbs free energy 338 ΔG ≠ of oil shale pyrolysis in air is greater than the activation energy E, and the Topo-chemical 339 method can advance spontaneously and continuously. In the process of calculating free enthalpy and free entropy by mapping ln -1 , it is found 358 that the data fitting curves of KAS and FWO methods are regular, as shown in Figure 12 to 13. 359 The experimental curves can be uniformly distributed near the fitting curve, and the fluctuation 360 is small. But the ln -1 curve obtained by Friedman method for calculating free enthalpy and 361 free entropy fluctuates greatly, as shown in Figure 14. The experimental curve is greatly 362 affected by the heating rate, and the correlation between the experimental points and the fitting 363 curve is also poor. This is because in the process of calculating activation energy by Friedman, 364 the data cannot be well fitted. The subsequent calculation is based on the previous calculation, 365 which is equivalent to magnifying the experimental error, so the data fitting degree is poor. 366 When the free enthalpy and free entropy are calculated by KAS and FWO methods, only when 367 the conversion rate is more than 95%, the obvious data dispersion phenomenon appears. This 368 is because the extrapolation method is not accurate enough to calculate the pyrolysis interval 369 of oil shale, which enlarges the temperature distribution interval of the second stage of oil shale 370 pyrolysis. However, this does not affect the accuracy of data processing, because this paper 371 focuses on the pyrolysis behavior of 40%-80% conversion in industrial state. 372 The ln -1 curves of free enthalpy and free entropy calculated by KAS and FWO methods 373 are shown in Figure 12 to 13. The stripping fitting curve appeared at the test points with higher 374 conversion. This is due to the uneven distribution of kerogen in the later stage of oil shale 375 pyrolysis and the errors in judging the pyrolysis process and burnout point. It also shows that 376 the calculation of burnout temperature by extrapolation method will enlarge the burnout 377 temperature to some extent. The results of KAS, FWO and Friedman methods show that the free enthalpy and free 381 entropy of oil shale pyrolysis at the second stage increase with the increase of heating rate, but 382 this trend is weak. This indicates that the process can be carried out spontaneously at high 383 temperature. As shown in Figure 15(a), the average free enthalpy calculated by KAS method is 384 66.98 kJ·mol -1 , and the average free enthalpy obtained by FWO method is 100.25 kJ·mol -1 . The 385 Friedman method has a low degree of data fitting in the calculation process, and the calculation 386 results fluctuate greatly, which has no value to reference. 387 As shown in Figure 15( Combined with the above process, the continuous combustion of oil shale will release 394 some internal energy to the environment. This measure of internal energy is free energy, which 395 leads to high temperatures in the environment. High temperature conditions will react with the 396 reaction system and transfer energy to the reaction system. When the energy transported is 397 greater than the activation energy, the reaction can proceed spontaneously. In the pyrolysis 398 process of Fuyu oil shale, the average activation free energy calculated by FWO method is 399 204.12 kJ·mol -1 , which is much larger than 133.59 kJ·mol -1 calculated by FWO method. The 400 average activation free energy calculated by KAS method is 271.82 kJ·mol -1 , which is also 401 much larger than the average activation energy calculated by KAS method in the second stage, 402 128.27 kJ·mol -1 . Therefore, the reaction can continue spontaneously at high temperature. 403

404
The product release characteristic index is a physical quantity describing the 405 characteristics of volatile matter release during oil shale pyrolysis. Because the second stage of 406 oil shale pyrolysis is the main stage of oil and gas generation, it is necessary to describe the 407 effect of temperature and heating rate on oil and gas release during Topo-chemical heat 408 pyrolysis oil shale, which provides an important reference for in-situ pyrolysis of oil shale. 409 In this paper, volatile release index I and reactive index R a are used to describe the 410 variation of pyrolysis product release characteristics with conversion and heating rate of oil 411 shale in Fuyu area under non-isothermal conditions [17][18][19] . In addition, the related calculation is 412 carried out at the conversion of 50% and 75% during the second stage of oil shale pyrolysis.
418 ∆T 1/2 -The temperature range corresponding to 50% conversion, K; also called peak width. 419 ∆T 3/4 -The temperature range corresponding to the conversion of 75%, K. 420 The above equation reflects the characteristics of volatile matter released by instantaneous 421 organic matter transformation in the process of oil shale pyrolysis to 50% and 75%. From Figure 16, it can be seen that the volatile release index in the second stage is 425 significantly affected by the heating rate. At the same heating rate, there is no difference 426 between the pyrolysis interval corresponding to 50% and 75%, but the product release 427 characteristic index corresponding to 50% conversion is more than 75%. This also indicates 428 that the higher the conversion, the more difficult the reaction will occur in the same pyrolysis 429 temperature interval. With the increase of heating rate, pyrolysis moves to high temperature 430 zone, and the volatile release index at the same conversion rate increases, too. When the heating 431 rate is 10 ℃/min, the volatile release index corresponding to 75% conversion is 0.3×10 -7 , and 432 when the heating rate reaches 50 ℃/min, it reaches 0.7×10 -7 . This is because the pyrolysis 433 process of kerogen is a process of chemical bond breaking and recombination. With the increase 434 of heating rate, the rate of interfacial reaction advancing into the interior of oil shale particles 435